This invention applies generally to the digitization of analog signals, and more particularly, to systems and methods for digitizing broadband transmission signals using multiple analog-to-digital converters and self calibration.
Typical community area or cable television (CATV) systems use distribution amplifiers throughout the transmission system to amplify broadband television signals.
The broadband television signals are transmitted over the distribution network from a headend in a downstream direction to subscribers. More complex CATV systems also include a reverse signal path from the subscribers to the headend. The reverse channel is generally used for system control, messaging, monitoring, pay-per-view events, video-on-demand, and other interactive television services. These two-way transmission systems generally use one spectrum of frequencies for forward transmissions (headend to subscriber) and another spectrum of frequencies for reverse transmissions (subscriber to headend). These systems typically use the higher of the frequency bands for forward transmissions and the lower frequency bands for reverse transmissions. Forward transmissions, for example, have generally been provided in the 54 to 550 Mhz band. Reverse transmissions have generally been provided in the 5 to 30 Mhz band.
Broadband communication systems, such as cable television (CATV) systems often utilize analog-to-digital converters (ADCs) to convert broadband analog signals to a digital format. Existing ADCs, however, have limitations and may not achieve the best digital approximation of the analog signal. To date, ADC manufacturers have been unable to successfully support effective digital conversions in higher frequency systems, such as those in the European market. Thus, the current cost, speed, and quality of conventional ADCs limit their effectiveness in applications, such as reverse CATV systems, that require the digitization of broadband signals.
One approach for achieving faster sampling rates while still maintaining good signal integrity is to use multiple lower cost ADCs in cooperation. The existing ADCs, working together in a system, can digitize a signal of greater bandwidth and with better integrity than could any one of the existing ADCs alone. This is accomplished by splitting the signal and applying it to separate ADCs. Each ADC then samples the signal in turn. The net sampling rate is the sampling rate of the individual existing ADCs multiplied by the number of ADCs working in cooperation. For example, if a sampling rate of 2fs is desired (where the sampling rate of each ADC is fs, the signal is split and applied to two different ADCs. Each ADC performs with sufficient linearity at the sampling rate of fs, but would have insufficient linearity at a sampling rate of 2fs. If, however, each ADC samples at fs, the ADC output signals can be recombined to provide an effective sampling rate of 2fs. Thus, a wider bandwidth signal can be digitized by using parallel path ADCs.
Use of multiple ADCs can, however, create problems. Most of these problems result from splitting the original signal, since varying environmental conditions, path delays, and timing issues can cause discrepancies between the signals on the parallel signal paths. For instance, one of the parallel signal paths may have more loss and/or greater delay than another. Further, the sampling clocks of the ADCs may have different delays so that the relative sampling times of the converters are less than ideal, causing further distortion. In addition, the individual ADCs may have different DC offsets.
Considering the high sampling rates and resolutions required for CATV applications, the conventional approach of interleaving samples using multiple ADCs imposes extremely rigid tolerances on circuits and components preceding the ADCs, on clocking components and layout, and on the ADCs themselves. The strict tolerances on these components are required to ensure that each ADC samples the signal in exactly the right phase relative to the other ADC(s). Furthermore, the gains or losses in each analog path must be precisely matched. A precise calibration procedure and manual adjustments are required to match various delays, gains, losses, and offsets. Once these conditions are met, there is still the possibility that the performance by the digitizer will degrade as component values change due to temperature variations, aging, or other environmental concerns. Such systems with sufficient stability and performance may be realizable for some applications, but the cost would be too great for many applications, such as typical reverse path CATV applications.
Thus, a need exists for lower cost, durable, stable systems and methods for achieving higher quality sampling of broadband analog signals at faster rates.